What's the easiest way to rotate around an edge that is not parallel to a primary axis?
Here is another way ( without effecting the pivot element ):


switch to (Pivot : cursor, Transform orientation : view, View : orthographic ).
select the element and press Shift+S->cursor to selected.
press Shift+(1 or 3 or 7) to go to one of the ortho views aligned to the selected element.
when in the desired view press Ctrl+I then rotate the object according to the view ( use RXX ..).